1. Field of the Invention
This invention relates to surface treatment of magnetic disk substrates, and more particularly to surface-treated aluminum alloy substrates for magnetic disks, with an oxide film free of defects such as cracks even in a high temperature heat treatment.
2. Description of the Prior Art
Heretofore, aluminum alloy sheets of a predetermined thickness which are obtained by machining and precision polishing are mainly resorted to as substrates for magnetic disks. The magnetic disks which are currently in use are mostly of the so-called coated type which is treated with chromate for improving deposition and corrosion resistance and coated with fine magnetic particles as a magnetic medium.
Recently, the magnetic memory disks have a trend toward a higher recording density, and for this purpose it is necessary to reduce the thickness of the magnetic medium and the gap space (hereinafter referred to as "spacing" for brevity) between a magnetic head and the magnetic medium.
Accordingly, the substrates for magnetic disks are required to meet the following conditions.
(1) The substrate surfaces after polishing should be of high accuracy to keep a uniform spacing and stablize the memory characteristics.
(2) The substrate surfaces should have high hardness and abrasive resistance in the case of contact-start/stop type disks.
(3) Substrates for .gamma.-Fe.sub.2 O.sub.3 media by a sputtering process or the like should be free of changes in the above-mentioned characteristics even after a heating condition of 300.degree.-400.degree. C., namely, should have high thermal resistance in heat treatments.
The conventional coated type disks with a relatively thick magnetic medium layer of 1-2.5 .mu.m and a large spacing of 1-2 .mu.m, employing substrates which are produced by cutting or polishing aluminum alloy of AA Standard 5086, and direct chromate treatment, satisfy the requirement (1) and involve no problem in particular with regard to the requirement (2). However, where higher recording density is aimed at, the target values for the thickness of magnetic medium and the spacing are smaller than 0.5 .mu.m and 0.4 .mu.m, respectively, though the existing substrates produced by cutting and polishing aluminum alloy sheets fail to guarantee the required surface accuracy as well as the required hardness and abrasive resistance.
In order to solve these problems of magnetic disks of high recording density, attempts have been made to harden the disk surface by forming electroless plated layer of Ni--P or an anodic oxide film on the surface of an aluminum alloy substrate, followed by mirror polishing and formation of a magentic film.
In the former case, 20-50 .mu.m thick Ni--P layer is plated on an aluminum alloy substrate and the plated surface is polished to meet the above-mentioned requirements (1) and (2). However, since direct plating on aluminum alloy is difficult, there arises a necessity for a scrupulous foundation treatment which makes the process complicate, and the electroless Ni--P plated layer which undergoes crystallization under heated condition above 200.degree. C. and as a result picks up magnetic property which is unsuitable as substrates for magnetic disks.
In the latter case, the anodic oxide film which is formed on an aluminum alloy substrate is polished to a thickness of 1-10 .mu.m to obtain the above-mentioned properties by a method as described, for example, in Japanese Patent Publication No. 53-037203. This method can meet the requirement (1) but normally involves a heat treatment at 350.degree.-400.degree. C. in the magnetic film forming process, in which the anodic oxide film is susceptible to cracks due to a difference in thermal expansion between the anodic oxide film and aluminum alloy unles the oxide film thickness is greater than 3 .mu.m. In addition, when polishing anodic oxide films to a thickness smaller than 3 .mu.m, it is extremely difficult to control the anodic oxide films on the entire surfaces of substrates to a thickness smaller than 3 .mu.m by polishing since the mirror-finished aluminum alloy substrates contain thickness irregularities greater than 3-5 .mu.m. On the other hand, if the spacing is minimized, there arises a problem that the magnetic head will hit upon and get into the substrate.
Although efforts have been made to solve the foregoing problems by increasing the thickness of the anodic oxide film, conventional methods failed to produce practically satisfactory magnetic disks due to the cracks which occur in the heat treatment of magnetic films. In this connection, Japanese Laid-Open Patent Application No. 58-016063 proposes a method of forming on an aluminum substrate a porous anodic oxide film with an allowance for polishing, washing the substrate with water, immediately polishing the oxide film to a mirror surface and into a thickness smaller than 8 .mu.m, washing the substrate with water again, and, after dehydration by a centrifugal dehydrator or by immersion in a hydrophilic organic solvent such as ethyl alcohol or the like for a water replacing treatment, drying at a temperature above 100.degree. C. to remove moisture completely from pores of the oxide film, storing the substrate in dried state until manufacture of magnetic disk. That is to say, water which is trapped in pores of the porous oxide film is completely removed to prevent pore sealing which would give rise to cracks upon heating in the magnetic film forming stage. In order to prevent sealing of the pores, the substrate requires strict control of dehydration not only during storage but also in the manufacturing process, and therefore invites complication of the manufacturing process.